Cesium ovens



July 4, 1961 E. F. GRANT ET AL CESIUM OVENS 2 Sheets-Sheet X Filed Jan.16, 1959 FIG. 3

FIGZ

EUGENE F. GRANT GORDON E. SIMPSON ARTHUR O. MCCOUBREY ROBERT S. BURITZINVENTORS BY W W, d/M HM 9 ATTORNEYS July 4, 1961 E, F, GRANT ETAL2,991,389

\CESIUM OVENS Filed Jan. 16, 1959 2 Sheets-Sheet 2 EUGENE F. GRANTGORDON E. SIMPSON ARTHUR O. MCOOUBREY ROBERT S. BURITZ INVENTORS BY WW9W g/ AM ATTOR N EYS United States Paten 2,991,389 CESIUM OVENS Eugene F.Grant, Marblehead Neck, Gordon E. Simpson, Melrose, and Arthur 0.McCoubrey and Robert S. Buritz, Topsfield, Mass., assignors to NationalCompany, Inc., Malden, Mass., a corporation of Massachusetts Filed Jan.16, 1959, Ser. No. 787,285 12 Claims. (Cl. 313-431) beam frequencystandard which utilizes the interaction of microwave energy of aparticular wave length with a beam of cesium atoms in afrequency-determining arrangement. Such apparatus is fully described inthe copending application of Jerrold R. Zacharias, et al., Serial No.693,- 104, filed October 22, 1957, for Molecular Beam Apparatus andassigned to the assignee of the present application. To understand thepresent invention, it is sufficient to state that during operation ofthe frequency standard, a flow of cesium atoms must be maintainedthrough an elongated beam tube, certain portions of which .areilluminated by microwave radiation. The cesium atoms, in gaseous form,are obtained from a reservoir of liquid cesium and evaporated by heatfrom a suitable source such as an electrical heating element. Afterevaporation from the reservoir, the cesium molecules pass through acollimator which aligns the molecules into beam for passage through thebeam tube.

Frequency standards of the above type have found increasing applicationin moving vehicles, such as airplanes, where they are subject to severeinertial forces including substantial vibration as well as completechange of orientation. Motion of this type has, in prior constructions,caused spillage of liquid cesium from its reservoir. Since the onlysupply of heat for evaporation of the charge is, in prior constructions,intimately associated with the reservoir, the cesium lost therefromremains in liquid form or reverts to the solid state and is therefore ofno further use in the frequency standard. The time during which thestandard may be operated before being recharged with cesium isconsequently materially reduced. The reliability of the apparatus isalso adversely affected by the:fact the spilled cesium may lie inportions of the beam tube and associated components of the frequencystandard in such manner as to cause serious malfunctions.

Accordingly, it is a principal object of our invention to provide animproved molecular beam source adapted to supply molecular cesium or thelike in a molecular beam frequency standard. A further object of theinvention is to provide a reservoir designed for incorporation in a beamsource of the above character and adapted to retain a charge of liquidcesium therein under conditions of intense inertial forces and changesor orientation. Another object of the present invention is to provide animproved beam source of the above character adapted to prevent leakageof liquid cesium into the frequency standard. Since the frequencystandard may be operated in moving vehicles such as airplanes or thelike, the source should be small in size and capable of lightweightconstruction. Simplicity of design is also a desired feature 2,991,389Patented July 4, 1961 objects and features of our invention will in partbe obvious and will in part appear hereinafter.

Our invention accordingly comprises the features of construction,combination of elements, and arrangements of parts which will beexemplified in theconstructions hereinafter set forth, and the scope ofthe inventionwill be indicated in the claims.

For a fuller understanding of the nature and objects ofthe'inventiomreference should be had to, the follow ing detaileddescription taken in connection with the accompanying drawings in which:

FIGURE 1 is a sectional view of a molecular beam source made accordingto our invention;

FIGURE 2 is a sectional view taken along line 2-2 of FIGURE 1, showingin detail a collimator structure which may be used with the cesium ovensof our invention;

FIGURE 3 is a view,.partly in section, taken along line 3-3 of FIGURE 1and illustrating in plan a baflie assembly used to prevent escape of thecesium charge from a reservoir of liquid cesium; and

FIGURE 4 is a sectional view of a second embodiment of a molecular beamsource embodying the principles of our invention.

In general, a molecular beam source embodying the features of ourinvention includes a reservoir containing a charge of liquid cesium anda heating coil which evaporates the cesium to provide the moleculescomprising the beam these being collectively referred to as an oven. Thereservoir has a baffle plate isolating it from other portions of thesource to prevent the flow of liquid cesium into these portions. Thebaffle plate is provided with narrow tubular members extendingtherethrough to permit the flow of cesium vapors from the reservoir, butthe tubes are so dimensioned and positioned as to prevent flow of theliquid metal through them even when the beam source is completelyinverted or subjected to intense vibration. The source also includes avalve adapted to cut olf the flow ofcesium vapors from the oven when thefrequency standard is not in use and a collimator which confinesthemolecules issuing from the source to a narrow well-defined beam. It willbe understood that while the apparatus tobe described below in greaterdetail has been incorporated in a molecular beam frequency standardusing. an atomic resonance of the cesium atom, the present invention isnot limited to use with this particular element. Thus, our beam sourcemay in many cases be used with frequency standards utilizing thecharacteristics of others of the alkaline metals or other materialshaving the proper molecular characteristics for use in frequencystandards, as noted in the above-identified application, Serial No.693,104. I

:FIGURE 1 illustrates a molecular beam source generally indicated at 2.The source includes a cesiumreservoir generally indicated at 4, a valvegenerally indicated at 6, and a collimator assembly generally indicatedat 8. During operation of the source, a cesium charge 10 is slowlyevaporated by a heating coil '11, and gaseous cesium atoms freed therebypass to the left (FIGURE 1) to the valve 6 and thence upwardly throughthe collimator assembly 8 where they are formed into a narrow beamprojected into other portions of the frequency standard (not shown).

7 More particularly, the source 2 has a cylindrical block generallyindicated at 12 recessed at one end to house the cesium reservoir 4.Achamber 13 in theother end of block 12 accommodates the valve 6. Thecollimator i assembly 8 is secured to block 12 and includes acollimainasmuch as it facilitates low cost fabrication. Other tor 14communicating with chamber 13. The latter is connected to reservoir 4 bya passage 16. A plug 18 brazed to block 12 at its right hand end sealsthe end 0 reservoir 4 and accommodates a filler tube 20.

Reservoir 4 comprises three chambers 22, 24 and 26 separated by bafiieplates 28,30 and 32. The latter may i i be suitably located in positionby annular spacers 34, 36 and 38. As seen in FIGURES l and 3,communication between chambers 22 and 24, 24 and 26, and between chamber26 and valve 6 is provided by a series of tubes 40 extending through andsecured to the respective baffles. The charge of liquid cesium isretained in chamber 22 and gaseous cesium is evaporatedtherefrom by heatfrom the coil 11, which, as seen in FIGURE 1, is disposed around thecollimator 14.

Valve 6 includes a valve stem 42 whose comically shaped nose 44 engagesa valve seat 45 comprising the end of passage 16. The head 46 of stem 42ridesin and is guided by a collar 48 secured in block 12, and a spring50 urges head 46 and stem 42 into'the open position against a diaphragm52'secured in collar 48. Diaphragm 52 transmits actuating forces to stem42 to close the valve and also provides a vacuum seal. Actuating forcesare provided in the construction illustrated by an actuating screw 54threaded through a cap 56 fastened to collar 48. A cup 58 of nylon orthe like serves as a buffer between screw 54 and diaphragm 52 tominimize wear on the diaphragm from turning of the screw. The flow ofcesium vapors through the valve 6 may thus be controlled by turningscrew 54 to move the valve nose 44 toward or away from the seat 45.

Both the diaphragm 52 and collar 48 are preferably of stainless steel,and proper care should be taken in securing the diaphragm to the collarto ensure the required vacuum-tight connection. This problem iscomplicated by the thinness of the diaphragm-about .0006 inch.Accordingly, the collar 48 includes two portions 48:: and 48b providedwith cooperating flanges 48c and 48d; the outer portions of thediaphragm 52 'aredisposed between these flanges. To assemble theseparts, the portions 48a and 48b are first clamped together with thediaphragm between them and preferably extending at least to the outeredges of the flanges. Next, an electric arc is struck between anelectrode and these outer edges, and the unit is rotated to permit thearc to move along the entire circumference thereof. The diaphragm S2 andflanges 48c and 48d are thus fused to form a unitary vacuum-tightstructure.

The collimator assembly 8 is housed within a sleeve 60 brazed to block12 and includes a pair of opposed support members 62 and 64 which are ofa generally semicylindrical shape. As best seen in FIGURES 1 and 2, thecollimator is disposed between and clamped in position by members 62 and64. The latter conform to the inner dimensions of sleeve 60 and aresecured within the sleeve by a retaining ring 66 brazed to these parts.In its preferable form, the collimator comprises alternate flat strips68 and corrugated strips 70 of nickel foil (FIGURE 2). Members 62 and 64are preferably of a good heat conducting material such as copper tomaintain the collimator 14 at a relatively high and uniform temperature.Sleeve 60 may be provided with a recess 72 (FIGURE 1) to facilitateconnection of the source to the beam tube of the molecular beamapparatus.

The beam source of FIGURE 1 is preferably encased in a cover 73ofsuitable heat insulating material and a second heater schematicallyindicated at 74 is disposed downstream (upward in FIGURE 1) fromthecollimator 14 and in contact with the sleeve 60. Accordingly,,thetemperatures within the beam source are dependent upon the temperaturesof the heating coil 11 and heater 74 and are substantially independentof the actualexternal ambient temperature.

The molecular beam frequency standard, of which the source 2 is acomponent, is internally evacuated to a hard vacuum, preferably mm.mercury or better. Therefore the chamber 22 and succeeding: portions ofthe source communicating with the recess 72 must be effectively sealedfrom the atmosphere. This may conveniently be accomplished by brazingthese parts together. Accordingly, the material of which the variousparts are made should be susceptible of joining by suitable brazingtechniques. It should also be relatively inert, particularly withrespect to cesium, and it should emit little or no contaminatingmaterial into the apparatus. Certain nickel copper alloys, specifically,the material sold by International Nickel Company under the same 403Monel have been found to meet these requirements. Accordingly, sleeve60, valve stem 42 and baffle plates 28, 30 and 32 are preferably of thismaterial. The block 12 is preferably of oxygen-free, highconductivitycopper in order to facilitate conduction of heat to the cesium charge 10in the chamber 22.

The tube 20, extending through plug 18 into reservoir chamber 22, isused to exhaust the apparatus and supply the cesium charge to thereservoir 4. After the source 2 is exhausted, it is preferably baked athigh temperature to drive out impurities, and then the temperature ofthe source is reduced to a low level, e.g., 0 C., and still under vacuumconditions, cesium from a supply thereof is distilled through the tube20 to condense in chamber 22. After a charge 10 of approximately 0.5gram, sufficient for many years operation, has accumulated in thechamber, the tube is pinched off as indicated at 20a.

During operation of the frequency standard, the cesium in chamber 22,which is maintained above its melting point by coil 11 and heater 74,rests on the inner wall of the block 12 when the source 2 is in theposition illustrated in FIGURE 1. Prior to the present invention,inversion or tilting of a source would cause fiow of the liquid from thereservoir into other parts of the apparatus. However, if the source 2 isstilted with the cesium thereby lying on baffle 28, escape of the liquidfrom the chamber 22 will be prevented by the tubes 40. These tubesproject into chamber 22 a sutncient distance so that their ends will beabove the liquid charge therein during tilting of the source 2. Thetubes are also disposed inwardly of the walls of the chambers asufiicient distance to maintain them above the liquid level when theaxis of reservoir 4 is horizontally oriented, as in FIGURE 1.Accordingly, the source may undergo complete rotation about any axiswithout appreciable loss of liquid cesium from the chamber 22 whilestill permitting the passage of cesium vapors fiom the reservoir by wayof the tubes 40.

By cascading the baifles 28, 30 and 32 and their associated tubes 40, wehave eliminated practically all possibility of loss of liquid cesiumfrom the reservoir 4. The small amount that may escape from chamber 22into chamber 24 during vibration of the source will find it still morediflicult to escape from the latter to chamber 26. An even smallerfraction of the liquid finding its way to chamber 26 will be able toescape from reservoir 4. Furthermore, as shown in FIGURE 1, the tubes 40project a substantially greater distance in the direction of chamber 22than they do into the chambers on the opposite sides of the respectivebaffles. The probability of liquid entering the shorter ends of thetubes is thus increased relative to the probability of entrance into thelonger ends. Accordingly, under conditions of vibration, inversion,etc., there will be a tendency toward net migration of liquid backthrough the tubes 40 to the chamber 22. For maximum flow discriminationby the tubes 40, the downstream ends 40a thereof should be as short asmay be compatible with eflicient fabrication of the baflle-tubeassemblies.

The liquid retention characteristics of the reservoir are furtherenhanced by the location and operation of the heating coil 11 and heater74. The temperatures of these heating units are controlled bythermostatic switches schematically indicated at 76 and 78,respectively. By way of example, coil 11 may be operated at a highertemperature than heater 74, the temperatures of these units being suchas to maintain the chamber 22 and the cesium charge 10 therein at atemperature of approximately 70 C., a level high enough to supplysutficient cesium vapor to the molecular beam apparatus. The temperatureof the collimator 40 will then be somewhat hotter, say 75 C., than thatof the cesium charge. Thus, the coolest location which the cesium maycontact is within the chamber 22 in the reservoir 4, and the temperatureof the reservoir 4 progressively increases as one moves from chamber 22to the left (FIGURE 1).

Should some of the liquid cesium escape to the left through the tubes40, it will tend to evaporate faster than the liquid within the chamber22, and gases contacting the escaped liquid will tend to condense at aslower rate than those in contact with the charge 10. Consequently,there is a tendency for the escaped cesium to migrate back into theregion of lowest temperaturechamber 22. Thus, over a substantial periodof time, there will be essentially no net flow of liquid from thechamber 22, even under severe environmental conditions encountered inaircraft use.

By suitable choice of materials, the reservoir 4 may be adapted forstill further protection against loss of the liquid charge. For example,copper, the preferred material of the block 12 is wetted by liquidcesium. Therefore, the charge spreads out to form a thin, adherent layeron the walls of chamber 22, as shown in FIGURE 1. This layer will bedislodged from the wall only by relatively intense vibration. The tubes40, on the other hand, are preferably of nickel (grade A or better),which is not wetted by cesium. Liquid cesium entering the tubes willtherefore remain in globular form rather than spread through the tubesby capillary action. To make full use of this feature, the tubes shouldhave small interior diameters, e.g., .02 inch. To provide the sameaction in chambers 24 and 26, spacers 34 and 36 should also be ofcopper.

In FIGURE 4, we have illustrated another embodiment of our invention inwhich a folded or reflex construction operates to conserve space andfacilitate the desired temperature distribution. The beam sourcegenerally indicated at 80 in FIGURE 4 has a cesium reservoir generallyindicated at 84, a valve generally indicated at 86 and a collimatorassembly generally indicated at 88. A cesium charge 90 in reservoir 84is evaporated by heat from a heating coil 92, and gaseous cesium atomsfrom the reservoir pass upwardly (FIGURE 4) to the valve 86 and thencedownwardly through the collimator assembly 88 where they are formed intoa narrow beam projected through an exit tube 94 communicating with otherportions of the frequency standard (not shown).

More particularly, the source 80 is enclosed by a cylindrical tube 96 towhich is brazed an end plate 98. Closure of the source is completed by aflexible diaphragm 99 secured between the tube 96 and the tubularhousing 100 of the collimator assembly 88. The reservoir 84 retains theliquid cesium charge 90 in a chamber 102 defined by tube 96, housing100, diaphragm 99 and a second diaphragm 104 also connected between tube96 and housing 100. A plurality of tubes 106 extending through diaphragm104 permit effusion of gaseous cesium from the chamber 102 whileinsuring retention of the liquid material therein in the mannerdescribed above. Tube 96 accommodates a filler tube 107 whose functionis similar to that of tube of FIGURE 1.

The valve 86 includes a valve nose 108 which is aflixed to end plate 98and is conically tapered to mate with a valve seat 110 at the upper endof the tubular housing 100. The relative movement of the nose and seatrequired for valve operation is provided by a dilferential screwmechanism generally indicated at 112. The latter comprises rings 114 and116 affixed to tubes 96 and 94, respectively, the rings being providedwith threaded portions 114a and 116a, one of which has a right handthread and the other a left hand thread. The threaded portions engagesimilarly threaded portions of a nut 118. Thus, as the nut is turned,rings 1.14 and 116 move toward or away from each other depending uponits direction of rotation.

Such relative movement is imparted by way of tubes 94 and 96 to thevalve nose 108 and seat 110, to open or to close valve 86. It will beapparent that flexibility of the diaphragms 99 and 104 is a desirablefeature to permit relative movement of their outer and inner edges withthe tubes 94 and 96 during valve operation. Accordingly, the diaphragmsare provided with the corrugated configurations shown in FIGURE 4.

The collimator assembly 88 includes a collimator 120 secured within thehousing and through which cesium atoms must pass on their way to thetube 94. It is preferably of the same general construction as thecollimator 14 of FIGURES 1 and 2.

The materials used in the beam source 80 should be the same as thosementioned above for the source 2. Thus tubes 94 and 96, plate 98,diaphragm 99 and valve 86 may be of a nickel-copper alloy such astheaforementioned 403 Monel. The upper diaphragm 104, however, ispreferably of copper in order to minimize the temperature gradientacross it. The tubes 106 are of nickel. The various parts may be joinedby brazing.

As seen in FIGURE 4, we have provided the beam source 80 with an outercasing 122 of heat insulating material. The source also has a secondheater 124 located at the bottom thereof in contact with the ring 116.The temperatures of the heating coil 92 and heater 124 are controlled bythermostatic switches 126 and 128, respectively, set to operate in likemanner to the switches 76 and 78 of FIGURE 2. Thus the coil 92 may beheated sufiiciently to maintain the collimator at a temperature of 75 C.The heater 124 is at a somewhat lower temperature than the coil 92 so asto keep the cesium charge 92 at a temperature of approximately 70 C.Accordingly, with the collimator 120 hotter than the charge 90, therewill be a net heat-induced migration of liquid cesium back to thechamber 102 in the manner described above. This eifect is aided by thefact that the highest temperature in the source 80 is that of the upperdiaphragm 104 on which liquid cesium displaced from chamber 102 byvibration will come to rest.

The tubes 106 of FIGURE 4 extend a sufiicient distance into the chamber102 and are disposed far enough from the tube 96 to inhibit flow ofliquid cesium therethrough. The ends of the tubes projecting on theother side of the diaphragm 104 are as short as practicable.Accordingly, the tubes 106 and diaphragm 104 function in the manner ofthe tubes and baffle-plates of the source 2 to offer much greaterresistance to flow from the chamber 22 than to reverse flow back intoit.

'It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efiiciently attained and,since certain changes may be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

We claim:

1. An improved molecular beam source adapted to supply gaseous moleculesof a substance from a liquid charge thereof, said source comprising, incombination, means forming a reservoir for said liquid charge, outletmeans and means forming a chamber between said reservoir and said outletmeans, a first wall between said reservoir and said chamber, a narrowtube passing through said wall and projecting into said reservoir beyondsaid wall adapted to pass said gaseous molecules to said chamber whilesubstantially impeding escape of said liquid from said reservoir.

2. The combination defined in' claim 1 including means for heating saidliquid in said reservoir to facilitate evaporation thereof.

3. The combination defined in-claim'2 including means for maintainingthe temperature in said chamber at a higher level than that of saidliquid charge in said reservoir whereby there is a tendency for liquidin said chamber to evaporatae and recondense in said reservoir.

4. The combination defined in claim 2 in which said tube projectsfarther into said reservoir than into said chamber, to thereby increasethe probability of a net liquid fiow from said chamber to saidreservoir.

5. The combination defined in claim 1 in which an interior surface ofsaid reservoir contacted by said charge is of a material subject towetting by said charge, and said tube is of a material not subject towetting by said charge.

6. The combination defined in claim 1 in which said reservoir includes asecond wall disposed between said first wall and said outlet means, saidsecond wall having a narrow tube passing through said second wall andprojecting beyond said second wall toward said first wall, whereby saidtube in said second wall may pass gaseous molecule toward said outletwhile substantially impeding passage of any liquid condensed in saidreservoir between said first wall and said second wall.

7. The combination defined in claim 6 in which said tubes projectfarther in the upstream direction toward said first wall than in thedownstream direction toward said outlet, to thereby increase theprobability of net liquid flow upstream through said tubes.

8. An improved molecular beam source adapted to supply gaseous moleculesof a substance from a liquid charge thereof, said source comprising, incombination, a housing, an end plate closing one end of said housing,outlet means including an exit tube disposed within said housinggand avalve having cooperating closure members second spaced walls extendingbetween said exit tube and said housing, a narrow gas tube projectingfrom said first wall into the interior of said reservoir and adaptedthereby to provide communication between said reservoir interior andsaid valve through the interior of said housing, whereby vapors mayreadily pass from said reservoir through said valve and exit tube whilesaid liquid charge is substantially sealed in said reservoir.

9. The combination defined in claim 8 in which said spaced walls aredeformable to permit said housing to move with said end plate duringsaid relative movement of said plate and said exit tube.

10. The combination defined in claim 8 in which said gas tube projects asufficient distance from said first wall and said housing to lie abovethe surface of said charge in all orientations of said source.

11. The combination defined in claim 8 including a collimator in saidexit tube downstream of said valve.

12. The combination defined in claim 8 including a heating coil aroundsaid housing and between said first wall and said end plate, saidheating coil being adapted to heat said liquid charge to facilitateevaporation thereof and provide a higher temperature between said firstwall and said end plate than in said charge whereby there is a tendencyfor any said liquid between said first wall and said end plate toevaporate and recondense in said reservoir.

References Cited in the file of this patent UNITED STATES PATENTS2,621,296 Thompson Dec. 9, 1952 2,714,665 Tunnell Aug. 2, 1955 2,808,510Norton Oct. 1, 1957

